My typical speed for flat ground is about 15mph. I can do 18 without pushing too hard and 20 when trying to keep closer to car traffic speed. However, on my ride into work in the early morning hours I often find I drop down to around 13mph and as low as 10 if I don't keep an eye on it. I think a lot of it for me is just that it's early in the morning and I know I give myself plenty of time to get to work.

Get a cheap computer to keep track of your speeds, or keep track of what times you pass certain points. I know that if I am turning onto one road at 5:15 I'm making good time, but at 5:20 I need to watch my speed a little closer.